Up in Montparnasse or Knock? (3)
I believe the answer is:
rap
'knock?' is the definition.
('rap' can be a synonym of 'knock')
'up in montparnasse or' is the wordplay.
'up' says the letters should be written backwards (in down clue: letters go upwards).
'in' indicates a hidden word.
'par' is hidden in the letters of 'montparnasse or'.
'par' written backwards gives 'RAP'.
(Other definitions for rap that I've seen before include "black music is a > blow" , "Knock - punishment - monologue to music" , "Criminal charge" , "Form of music with spoken lyrics" , "Sharp blow; criticism" .)